Novel binder oligomers in accordance with the present invention provide flexibility to photo-imaged photoresists and substantially improve performance of solder mask-forming photoresists when exposed to conditions of nickel/gold plating. While most important benefits of the binder oligomer are seen in solder mask-forming photoresists, flexibility is desirable in all photoresist compositions, including primary imaging photoresists.
A solder mask-forming photoresists forms a hard, permanent layer on a printed circuit board. A solder mask must be hard, durable, and resistant to chemicals, such as organic solvents and strong acids and bases. In this regard a solder mask should meet the standards of IPC/ANSI SM-840B Table 12 tests, Summary of Criteria for Qualification/Conformance (Institute for Interconnecting Circuits). Herein, a solder mask is defined as a layer which meets those tests. Examples of solder mask-forming compositions that are developable in alkaline aqueous solutions are U.S. Pat. Nos. 5,229,252 and 5,364,736, the teachings of each of which are incorporated herein by reference.
Previously, solder was applied directly to exposed pads of the copper electronic circuitry traces for electrical connection to surface mounts and the like. Problems with soldering directly to the circuitry traces include expense of the solder, toxicity of the solder, copper migration into the solder, and tarnishing of the solder. An improved process involves nickel/gold plating of the pads in preparation for electrical connection. The exposed pads are initially plated with nickel by an electroless process to a thickness of between about 100 and about 300 micro inches. Then a thin layer, e.g., 6 micro inches, of gold is plated over the nickel by an electroless process. Nickel/gold plating is advantageous in that the layers are planer and the nickel gold protects the copper from migrating. Also, the gold does not tarnish.
However, the electroless plating processes are harsh, requiring solder masks to withstand to the plating conditions. Particularly, nickel electroless plating is typically conducted at about 90.degree. C. for 15 minutes. Often, lifting of the solder mask in the regions around the pads is observed.
Lower molecular weight binder polymers of solder mask-forming photoimageable compositions tend to shrink less during processing and result in less lifting of the solder mask. However, lower molecular weight binder polymers tend to be tacky, a trait undesirable because contact imaging, i.e., direct contact between the artwork and the photoimageable composition, is required for maximum resolution.
Higher molecular weight binder polymers reduce tack but shrink more, resulting in a greater amount of solder mask lifting during nickel/gold plating.
Many current solder mask-forming photoresists are based on a styrene-maleic anhydride binder polymer. Because of the inflexibility of styrene/maleic anhydride polymers, solder mask-forming photoresists based on such polymers tend to hold up poorly under conditions of nickel/gold plating.
The present invention is directed to flexible photopolymerizeable binder oligomers which reduce the effect of property trade-offs, such as tack versus lifting in a solder mask composition exposed to nickel/gold plating.